1. Field of the Invention
The present invention relates generally to multilevel phase change memory, and in particular to a ring heater for a phase change memory device.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that may be electrically switched between a structural state of generally amorphous local order and a structural state of generally crystalline local order or among different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline order. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that phase or physical state and the resistance value associated thereto are retained until changed by another programming event. The state is unaffected by removing electrical power.
A multilevel phase change memory is capable of being programmed to more than the two conventional states, termed set and reset. In order to form the multilevel memory, it is desirable to provide a cell which is capable of being gradually programmed so that more than two programmable states can be provided. This generally means that the plot of resistance versus current for the cell should have a gradual slope.